The chemistry of retarding scorch is necessarily linked to the crosslinking chemistry, such that different scorch retarders are required for each type of crosslinking system. FKM elastomers are believed to crosslink via a multistage process when bisphenol curatives are used, the first stage of which is dehydrofluorination to generate electron-deficient double bonds, followed by a Michael addition of bisphenol to generate a grafted bisphenol, followed by a second Michael addition of grafted bisphenol to generate a crosslink. Both the dehydrofluorination and the Michael addition reactions are usually catalyzed by quaternary ammonium or phosphonium salts. It has been observed that most of the bisphenol grafts before any of the grafted bisphenol adds to a second FKM chain (which creates crosslinks). Adding more bisphenol to a formulation increases the scorch time, while also increasing the final crosslink density. This observation led to a search for scorch delay additives that function by competing with bisphenol to delay the onset of crosslinking in FKM. U.S. Pat. No. 5,756,588 to Kolb and Jing (assigned to 3M) is the first patent to successfully utilize this mechanism to extend scorch delay of bisphenol-cured FKM. U.S. Pat. No. 5,756,588 teaches that electron-withdrawing substituents on the benzene ring of phenol are particularly desirable in scorch retarding monophenols.
Table 1 summarizes key findings reported in U.S. Pat. No. 5,756,588; these data also support the contention within U.S. Pat. No. 5,756,588 that electron-withdrawing substituents on a phenol ring increase the effectiveness of monophenol scorch inhibitors. According to the data of Table 1, p-cyanophenol (also known as 4-hydroxybenzonitrile) has the best balance of properties of all the tested compounds, as it simultaneously accelerates the cure by a factor of 1.5, and delays the onset of scorch by a factor of 2.08 under the test conditions.
Another relevant prior art patent is U.S. Pat. No. 4,446,270. This patent describes certain particular monophenols which also bear allyl functional groups. The purpose of these materials is to attach (xe2x80x9cgraftxe2x80x9d) the allyl functional group to an FKM polymer chain, as a means of increasing crosslink density and hardness. Some of the materials described in this patent may also function to delay the onset of scorch in bisphenol-cured fluoroelastomers, but this was not an objective of U.S. Pat. No. 4,446,270. Furthermore, this patent cites no evidence that an effect on scorch delay actually occurred.
(All monophenols listed were compared in an identical FKM basis formulation, with constant molar level of monophenol additive, except for the control; see U.S. Pat. No. 5,756,588 for details.)
Many of the particular monophenols described U.S. Pat. No. 5,756,588 in U.S. Pat. No. 5,756,588 are either toxic, or are relatively expensive. Among the monophenols described in U.S. Pat. No. 5,756,588, perhaps the least toxic chemical is methylparaben, which is commonly used as a preservative in various topical ointments. Unfortunately, methylparaben slows the cure of FKM while delaying the onset of scorch. Methylparaben has also been found to be difficult to disperse properly; in many instances formulations containing methylparaben must be re-milled to achieve proper dispersion.
A particular scorch inhibiting monophenol additive, butylparaben, has been found which has surprising cure system activity compared to methylparaben, an analogous chemical that was described in the prior art U.S. Pat. No. 5,756,588. Butylparaben is also effective as a processing aid, in a variety of different elastomer systems. Although the prior art U.S. Pat. No. 5,756,588 teaches that improved activity corresponds to electron withdrawing substituents on the phenol ring, the butyl group is relatively electron-releasing, so it is surprising that butylparaben increases rather than decreases both cure rate and efficiency in inhibiting scorch, especially when compared to methylparaben. Propylparaben has intermediate activity, and is not in and of itself highly desirable over methylparaben.
It was suspected that the increased activity of butylparaben over methylparaben and propylparaben is due to its lower melt point; therefore various melt blends of butylparaben, methylparaben, and propylparaben were prepared in order to map out the melt point of the blends versus concentration. It was found that low-melting eutectic mixtures form which remain fluid for extended times at temperatures as low as 50xc2x0 C. The very lowest melting/solidifying mixtures contain more than 30% by weight of butylparaben, and these mixtures would be particularly suitable for liquid injection systems for scorch delay additives into an internal mixer or extruder. and are not as desirable as the mixtures that melt/solidify around 75xc2x0 C. Among these mixtures, the most economical mixtures which melt/solidify around 75xc2x0 C. consist of approximately 10% butylparaben, 40% propylparaben, and 50% methylparaben.
When said eutectics are cooled, large methylparaben crystals formed which made the resultant flakes difficult to disperse into FKM elastomer, although once the dispersion was accomplished, these eutectic mixtures exhibited equivalent effectiveness to butylparaben at lower cost. Subsequent experiments showed that addition of colloidal silica interfered with recrystallization of the eutectic to a sufficient extent that large crystals did not form during cooling. Eutectic mixtures of methylparaben, propylparaben, and a minor portion of butylparaben can be mixed with various colloidal mineral fillers, then cooled to powder, flakes, pellets, or pastilles which are not dusty, and which readily disperse into typical FKM compounds. These solidified, colloidal mineral-containing eutectic paraben mixtures contain only small methylparaben crystals, which leads to rapid uniform dispersion into FKM, even at processing temperatures slightly below the melting temperature of the eutectic.